Phenotypic damage and transcriptomic responses of flower buds in rapeseed (Brassica napus L.) under low-temperature stress

Low temperature occurs frequently in the early spring during the flowering period of rapeseed (Brassica napus L.), which always results in yield loss of rapeseed. However, very little is known about the effects of cold on rapeseed during the early flowering stage. This study investigated the phenotypes and transcriptome profiles of two B. napus inbred lines (10B and GZH) in the early flowering stage. The ratio of pollen viability and stigma receptivity of cold-treated buds were detected on the flowering day, which were dramatically decreased before the binucleate period. Pollination experiments ascertained the same contributions to yield loss between cold damaged male and female parts. Furthermore, a total of 1396 differentially expressed genes (DEGs) were identified in flower buds under different cold treatments via RNA-seq. These genes were significantly enriched in biological processes related to photosynthesis, circadian rhythm, inositol metabolism, phenylpropanoid biosynthesis, response to temperature stimulus. According to gene expression patterns, functional annotations, and nucleotide variations, 104 DEGs were predicted as conservative genes responding to cold stresses in the buds of rapeseed, which were associated with abiotic stress response, photosynthesis, circadian rhythm, metabolisms (phenylpropanoid, polyamine, sugar, and lipid metabolisms), and gamete cell development. Based on these findings, a hypothetical model of rapeseed buds under low temperature conditions is proposed, in which rapeseed bud responds to low temperature through the CBF-dependent pathways and disordered biological metabolisms. This study will be useful for elucidating the molecular basis of the cold responsive mechanism of young buds in B. napus.